Al2 O3 and Y2 O3 containing silicon nitride having high mechanical strength at high temperatures

ABSTRACT

A method of making a shaped part of sintered silicon nitride is disclosed which includes preparing a mixture of Si 3  N 4  having a BET specific surface area in the range of from 2 to 15 m 2  /g, having an O 2  content of &lt;1.5% by weight, a β-form content of &lt;2% by volume with finely divided Y 2  O 3 , Al 2  O 3  or HfO 2  and/or ZrO 2 , the total additive content being in the range of from 6 to 13% by weight, based on the total weight of the mixture, then mixing and milling mixture in a liquid dispersion medium, drying and agglomerating the suspension so produced, subsequently pressing, injection molding or redispersing and casting the agglomerated material obtained to make shaped parts, and finally sintering the shaped parts at temperatures between 1725° and 1850° C. under nitrogen for a period of up to 2 hours. The shaped parts produced according to the method have high mechanical strength and include at least 87% by weight Si 3  N 4  in the crystalline phases of the α- and/or β-form, up to 13% by weight of an additive combination of Al 2  O 3  and Y 2  O 3 , the Y 2  O 3  /Al 2  O 3  weight ratio being in the range of from 1.1 to 3.4, and additionally from 0 to 1.0% by weight of HfO 2  and/or ZrO2. The shaped parts have a density of greater than 98% of the theoretically possible density and a bending strength of ≧850 MPa at room temperature and ≧800 MPa at a temperature of 1000° C.

This application is a division of application Ser. No. 08/534,059, filed Sep. 26, 1995, now U.S. Pat. No. 5,556,815, which is a continuation of Ser. No. 08/170,921, filed Dec. 21, 1993, now abandoned.

The present invention relates to shaped parts of sintered silicon nitride ceramic which have high strength both at room temperature and at high temperatures and which contain Si₃ N₄ in the crystalline phases of the α and/or β forms.

Silicon nitride and components produced therefrom by sintering have already been described in numerous publications. From JP-A-59/18165 it is known, for example, that a mixture of Si₃ N₄ with Y2O₃ and Al₂ O₃ in a weight ratio of 94 to 3 to 3 can be sintered under gas pressure at temperatures above 1900° C. and that high strengths at room temperature can thus be achieved. The weight ratio of Y₂ O₃ to Al₂ O₃ according to this publication is 1; however, in the case of production via a two-stage pressure sintering cycle at very high temperatures between 1900° and 1950° C. in combination with a very long sintering time of a total of 4 hours, this leads however to a microstructure which is too coarse in respect of its crystallite size to still be suitable for specific applications in high performance technology.

From JP-A-63/139057 it is known that Al₂ O₃ and Y₂ O₃ in a ratio of 1:1 can be admixed with silicon nitride (5% by weight and 5% by weight), but the presence of aluminum nitride in an amount of 2.1% by weight as additionally required according to this publication leads to the formation of sialons, which impairs the technical properties, in particular the strength, of the silicon nitride parts.

EP-A-197 548 discloses that from 0.5 to 4% by weight of Al₂ O₃ and from 4 to 8% by weight of Y₂ O₃ can be used as additives to silicon nitride without needing to adhere to a particular weight ratio of the two additives. According to this publication, however, only from 85 to 97% of the theoretically possible density is achieved in the shaped parts.

The presence of HfO₂ as an additive in silicon nitride is known from a further Japanese publication JP-A-31/53574. According to this publication, however, the HfO₂ combines with yttrium to give Y₂ Hf₂ O₇.

It is an object of the present invention to provide an additive combination which makes it possible to further improve the strength properties of sintered shaped parts of silicon nitride at room temperature and simultaneously also at temperatures above 800° C., and to provide a corresponding production process for components of silicon nitride having exceptional high-temperature strength.

This object is achieved by shaped parts of the generic type mentioned in the introduction, the characteristic features of which are that they comprise at least 87% by weight of silicon nitride and up to 13% by weight of an additive combination of Al₂ O₃ and Y₂ O₃, the Y₂ O₃ /Al₂ O₃ weight ratio being in the range of from 1.1 to 3.4, that they additionally contain from 0 to 1.0% by weight of HfO₂ and/or ZrO₂, that the shaped parts have a density of greater than 98% of the theoretically possible density and that they have a flexural strength of >850 MPa at room temperature and of ≧800 MPa at a temperature of 800° C.

Contrary to the general view, it has been possible to show that shaped parts of Si₃ N₄ ceramic having relatively high Al₂ O₃ contents can be produced without the glass phase crystallizing out and have good strength both at room temperature and also at elevated service temperatures with a simultaneously high fracture toughness. In accordance with the invention this is achieved by components of HfO₂ and/or ZrO₂ in the amorphous phase and/or the setting of suitable Y₂ O₃ /Al₂ O₃ ratios. The materials dissolved in the amorphous phase raise the transition temperature T_(G) of the amorphous phase and/or its viscosity-temperature behavior above T_(G) is favorably affected in the direction of higher strengths of the shaped parts at high temperatures.

The microstructure of a Si₃ N₄ ceramic is made up of rod-shaped Si₃ N₄ crystals and the grain boundary phase. The crystals are normally present in a disordered, purely random arrangement which is described as isotropic microstructure. For characterization of the microstructure test pieces are ground, etched and then optically examined with a scanning electron microscope to produce photographs at magnifications of 5000 and 10,000. The photographs are then evaluated semi-automatically. For each test piece about 800 to 1500 grains must be evaluated, in each case the longest diagonal of a grain being defined as the grain length L and the shortest diagonal being defined as the grain width W. The aspect ratio A is the ratio of L to W. To determine the aspect ratio at least 40 grains per test piece having grain edges as close to parallel as possible and grain lengths as long as possible are selected, so as to level out the influence of the sectioning effect.

The shaped parts of the invention preferably possess a microstructure which has a fine grain arrangement of silicon nitride having more than 3·10⁶ grains/mm² with an average width of the silicon nitride crystallites of less than 0.4 μm, a maximum grain width of at most 2 μm and an aspect ratio of greater than 8 within a finely divided amorphous phase, the amorphous grain boundary phase between the Si₃ N₄ grains being no thicker than 0.1 μm.

The fracture toughness K_(IC) of the shaped parts of the invention is preferably greater than 9 MPa·√m, their mechanical flexural breaking strength (4 point bend test in accordance with DIN 51 110) at room temperature is greater than 950 MPa and their mechanical flexural breaking strength above 800° C. is greater than 800 MPa. Because of the high sintered bulk density of ≧98% of the theoretically possible density the modulus of elasticity is in the range of from 290 to 320 GPa. With the relatively low coefficient of thermal expansion of 3.1·10⁻⁶ /K and a thermal conductivity of 20 W/m·K the shaped parts of the invention possess good thermal shock resistance, quantified by a critical temperature difference of ≧800 K.

The components of the invention are particularly suitable for use in machine and plant construction and as valves in combustion engines for automobiles.

In the process for producing the shaped parts of silicon nitride according to the invention, a mixture of Si₃ N₄ having a specific surface area of from 2 to 15 m² /g, an O₂ content of <1.5% by weight and a proportion of β form of <2% by volume with finely divided Y₂ O₃, Al₂ O₃ or HfO₂ and/or ZrO₂ is first prepared. The total additive content should, according to the invention, be between 5 and 13% by weight, based on the total weight of the mixture. In particular, the additive content should be in the range of from 8 to 10% by weight. The mixture is then mixed and milled in a liquid dispersion medium (water or organic solvent) and the suspension so produced is dried and agglomerated in, for example, a spray dryer. The agglomerated material obtained is then made into shaped parts by pressing, injection molding or redispersing and casting and then sintering under gas pressure at temperatures between 1725° and 1850° C. and at N₂ pressures between 2 and 8 MPa over a period of up to 2 h.

The examples below are intended to illustrate the invention in more detail for those skilled in the art, without however limiting it to the particular embodiments.

EXAMPLES 1 AND 2 (COMPARATIVE EXAMPLES)

Powder mixtures of 87.7% by weight of Si₃ N₄ (98% of α--Si₃ N₄ and 2% of β from; O₂ content<1.5% by weight, BET specific surface area=11 m² /g), 2.3% by weight of Al₂ O₃ and 10% by weight of Y₂ O₃ were prepared. This corresponds to a total additive content of 12.3% by weight and a Y₂ O₃ :Al₂ O₃ weight ratio of 4.35. These mixtures were intensively mixed and milled in batches of 400 g in an agitated ball mill using Al₂ O₃ milling media and isopropanol as dispersion medium for a period of 2 h. The suspension was dried in a rotary evaporator and subsequently sieved through a sieve having a mesh size of 250 μm. The powders were subsequently isostatically pressed at 300 MPa to give compacts.

In Example 1, sintering was carried out without pressure in a 0.1 MPa N₂ atmosphere by raising the temperature to 1800° C. over a period of one hour, maintaining it at this value for one hour and then cooling back down to room temperature over a further three hours.

In Example 2, sintering was carried out under a gas pressure of 2 MPa over a period of 30 min. The results of Examples 1 and 2 are shown in Table 1. The use of gas pressure has improved the high-temperature strengths. The high-temperature strengths of both samples were below 700 MPa with a relatively high fracture toughness of K_(IC) =9.2 and 10.1 MPa·√m respectively.

EXAMPLES 3 TO 5

Powder mixtures of 87.7% by weight of Si₃ N₄ (98% of α--Si₃ N₄ and 2% of β form; O₂ content<1.5% by weight, BET=11 m² /g) were prepared with various total contents of sintering additives and various Y₂ O₃ :Al₂ O₃ ratios.

    ______________________________________                                                        Total additives                                                                         Y.sub.2 O.sub.3 :Al.sub.2 O.sub.3                      ______________________________________                                         Example 2 (comparative                                                                          12.3       4.35                                               example)                                                                       Example 3        10.00      3.5                                                Example 4 (invention)                                                                           8.00       2.0                                                Example 5        6.00       1.0                                                ______________________________________                                    

As described in connection with Example 1, the samples were mixed, milled, dried and made into shaped parts and sintered under gas pressure. The results are summarized in Table 2.

Sufficiently good compaction was obtained only in Examples 4 and 2, with Example 4 corresponding to the invention and being distinguished from Example 2 (comparative example according to the prior art as described by Boberski et al., Mechanical Properties of Gas Pressure Sintered Silicon Nitride, in Proceedings of 4th internat. Symp. of Ceramic Material and Components for Engines, Goteborg 1991) by the total additive content being reduced and the Y₂ O₃ :Al₂ O₃ ratio being lowered to 2.

The flexural breaking strength of the sample in Example 4 is 800 MPa at 1000° C. Furthermore, these samples have a high K_(IC) value of 10.5·√m MPa. The individual features of the structure are shown in Table 2.

Besides Si₃ N₄ there is an amorphous grain boundary phase. The exceptionally good high-temperature properties were achieved by the selection of suitable additive ratios and the use of a customized gas-pressure sintering program, which lead to the formation of the structure of the invention.

EXAMPLES 6, 7 AND 8

To vary the total amount of additive while maintaining a constant Y₂ O₃ /Al₂ O₃ ratio, powder mixtures of Si₃ N₄ (98% of α--Si₃ N₄ and 2% of β--Si₃ N₄, O₂ content of <1.5% by weight; BET=11 m² /g) having different total contents but a constant ratio of Y₂ O₃ to Al₂ O₃ =2 are prepared. In addition, to change the liquid phase (viscosity) during sintering, 0.2% by weight of ZrO₂ or 0.4% by weight of HfO₂ and, for comparison, 0.2% by weight of CaO were added.

    ______________________________________                                                      Addition to Total additive                                                     glass-phase content                                               ______________________________________                                         Example 6      0.2% by weight CaO                                                                            6.0% by weight                                   Example 4 (invention)                                                                         --             8.0% by weight                                   Example 7 (invention)                                                                         0.4% by weight HfO.sub.2                                                                     10.4% by weight                                   Example 8 (invention)                                                                         0.2% by weight ZrO.sub.2                                                                     12.5% by weight                                   ______________________________________                                    

As described in Examples 1 and 2, the samples were mixed, milled, dried and pressed to give shaped parts and sintered under gas pressure. The results are summarized in Table 3. Sufficiently good compaction was not achieved in Example 6; this means that with the selected compositions and gas-pressure sintering conditions sufficient compaction to guarantee good strength could not be achieved. For the low additive content of 6% by weight, therefore, other compaction conditions must be selected. The addition of HfO₂ (Example 7) with a somewhat increased total additive content has led to both excellent compaction and to very good high-temperature properties and gave 869 MPa at 1000° C. The sample was also distinguished by the Si₃ N₄ being present in both the α and the β forms. HfO₂ is present in solution in the glass phase, since no hafnium-containing crystalline phase in addition to Si₃ N₄ could be indentified in the X-ray diffraction spectrum. The addition of ZrO₂ (Example 8) also leads to good high-temperature properties.

                  TABLE 1                                                          ______________________________________                                         Composition, sintering conditions and                                          results for the prior art                                                      Example              No. 1     No. 2                                           ______________________________________                                         Composition  % by weight!                                                      Si.sub.3 N.sub.4     87.71     87.71                                           Al.sub.2 O.sub.3     2.28      2.28                                            Y.sub.2 O.sub.3      10.01     10.01                                           Total additive content                                                                              12.29     12.29                                           Y.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                                   4.4       4.4                                             Sintering conditions                                                           Temperature  °C.!                                                                            1800      1750                                            N.sub.2 pressure  MPa!                                                                              0.1       2                                               Holding time  h!     1         0.5                                             Density  % of theoretical                                                                           98.5      98.9                                            density!                                                                       Crystalline phase composition                                                                       β-Si.sub.3 N.sub.4                                                                  β-Si.sub.3 N.sub.4                         Flexural breaking strength*  MPa!                                              at 25° C..sup.1)                                                                             720       771                                             at 1000° C..sup.2)                                                                           545       699                                             Fracture toughness K.sub.IC                                                                         9.2       10.1                                             MPa · √m!                                                     Microstructural parameters:                                                    Aspect ratio         6.0       7.5                                             Number of grains per unit                                                                           1.0       4.0                                             Area  10.sup.6 /mm.sup.2!                                                      Average grain width  μm!                                                                         0.6       0.5                                             ______________________________________                                          *The flexural breaking strength was determined in accordance with DIN 51       110                                                                            K.sub.IC : was determined by the notched beam method in accordance with        DIN 51 109 (notch depth = 1/3of the specimen thickness)                        .sup.1) average of at least 6 measured values                                  .sup.2) average of at least 3 measured values                            

                  TABLE 2                                                          ______________________________________                                         Composition, sintering conditions and                                          results for Examples 3 to 5                                                    Example           No. 3    No. 4    No. 5                                      ______________________________________                                         Composition  % by weight!                                                      Si.sub.3 N.sub.4  89.95    92.00    94.00                                      Al.sub.2 O.sub.3  2.28     2.67     3.00                                       Y.sub.2 O.sub.3   7.77     5.33     3.00                                       Total additive content                                                                           10.00    8.00     6.00                                       Y.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                                3.5      2.0      1.0                                        Sintering conditions                                                           Temperature  °C.!                                                                         1800     1725     1850                                       N.sub.2 pressure  MPa!                                                                           2        8        8                                          Holding time  h!  1.5      1.5      0.5                                        Density  % of theoretical                                                                        94.6     99.99    97.9                                       density!                                                                       Crystalline phase composition                                                                    β-Si.sub.3 N.sub.4                                                                 β-Si.sub.3 N.sub.4                                                                 β-Si.sub.3 N.sub.4                    Flexural breaking strength*  MPa!                                              at 25° C..sup.1)                                                                          798      983      724                                        at 1000° C..sup.2)                                                                        601      800      579                                        Fracture toughness K.sub.IC                                                                      8.9      10.5     8.2                                         MPa · √m!                                                     Microstructural parameters:                                                    Aspect ratio      8.0      9.7      8.5                                        Number of grains  2.0      4.5      3.5                                         10.sup.6 /mm.sup.2!                                                           Average grain width  μm!                                                                      0.5      0.2      0.3                                        ______________________________________                                          *The flexural breaking strength was determined in accordance with DIN 51       110                                                                            K.sub.IC : was determined by the notched beam method in accordance with        DIN 51 109 (notch depth = 1/3of the specimen thickness)                        .sup.1) average of at least 6 measured values                                  .sup.2) average of at least 3 measured values                            

                  TABLE 3                                                          ______________________________________                                         Composition, sintering conditions and                                          results for Examples 6 to 8                                                    Example           No. 6    No. 7     No. 8                                     ______________________________________                                         Composition  % by weight!                                                      Si.sub.3 N.sub.4  93.80    89.61     87.51                                     Al.sub.2 O.sub.3  2.00     3.33      4.10                                      Y.sub.2 O.sub.3   4.00     6.66      8.20                                      Addition          0.20 CaO 0.4 HfO.sub.2                                                                            0.2 ZrO.sub.2                             Total additive content                                                                           6.2      10.4      12.5                                      Y.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                                2.0      2.0       2.0                                       Sintering conditions                                                           Temperature  °C.!                                                                         1800     1750      1850                                      N.sub.2 pressure  MPa!                                                                           2        8         2                                         Holding time  h!  0.5      0.5       1.5                                       Density  % of theoretical                                                                        92.0     99.1      98.5                                      density!                                                                       Crystalline phase composition                                                                    β-Si.sub.3 N.sub.4                                                                 11%       β-Si.sub.3 N.sub.4                                              β-Si.sub.3 N.sub.4                                                        89%                                                                            β-Si.sub.3 N.sub.4                             Flexural breaking strength*  MPa!                                              at 25° C..sup.1)                                                                          707      1045      853                                       at 1000° C..sup.2)                                                                        566      869       805                                       Fracture toughness K.sub.IC                                                                      7.6      9.5       10.0                                       MPa · √m!                                                     Microstructural parameters:                                                    Aspect ratio      7.5      12.2      11.5                                      Number of grains  4.0      6.1       5.0                                        10.sup.6 /mm.sup.2!                                                           Average grain width  μm!                                                                      0.45     0.15      0.25                                      ______________________________________                                          *The flexural breaking strength was determined in accordance with DIN 51       110                                                                            K.sub.IC : was determined by the notched beam method in accordance with        DIN 51 109 (notch depth = 1/3of the specimen thickness)                        .sup.1) average of at least 6 measured values                                  .sup.2) average of at least 3 measured values                             

What is claimed is:
 1. A process for producing a shaped part of sintered silicon nitride, comprising the steps offirst preparing a mixture of Si₃ N₄ having a BET specific surface area in the range of from 2 to 15 m² /g, having an O₂ content of <1.5% by weight, a β-form content of <2% by volume with finely divided Y₂ O₃, Al₂ O₃ or HfO₂ and/or ZrO₂, the total additive content being in the range of from 6 to 13% by weight, based on the total weight of the mixture, then mixing and milling mixture in a liquid dispersion medium, drying and agglomerating the suspension so produced, subsequently pressing, injection molding or redispersing and casting the agglomerated material obtained to make shaped parts, and finally sintering said shaped parts at temperatures between 1725° and 1850° C. under nitrogen for a period of up to 2 hours, whereby the shaped part consists essentially of at least 87% by weight Si₃ N₄ in the crystalline phases of the α- and/or β-form, up to 13% by weight of an additive combination of Al₂ O₃ and Y₂ O₃, the Y₂ O₃ /Al₂ O₃ weight ratio being in the range of from 1.1 to 3.4, and additionally from 0 to 1.0% by weight of HfO₂ and/or ZrO₂, wherein the shaped part has a density of greater than 98% of the theoretically possible density and its bending strength is ≧850 MPa at room temperature and ≧800 MPa at a temperature of 1000° C.
 2. Process as claimed in claim 1, wherein the total additive content in the pre-sintered mixture is in the range of from 8 to 10.5% by weight.
 3. The process as claimed in claim 1, wherein the shaped part has a fracture toughness K_(IC) of ≧8 MPa·√m.
 4. The process as claimed in claim 1, wherein the silicon nitride ceramic shaped part has a number of grains per unit area of greater than 3·10⁶ grains/mm².
 5. The process as claimed in claim 4, wherein the grains have an average grain width of ≦0.4 μm.
 6. The process as claimed in claim 5, wherein the grains have a maximum grain width of ≦2 μm.
 7. The process as claimed in claim 5, wherein the grains have an aspect ratio of ≧8.
 8. The process as claimed in 1, wherein the shaped part has an amorphous grain boundary phase between the Si₃ N₄ grains with a maximum thickness of 0.1 μm.
 9. The process as claimed in claim 1, wherein the shaped part has a fracture toughness K_(IC) of ≧9 MPa·√m.
 10. The process as claimed in claim 1, wherein the shaped part has a mechanical flexural breaking strength at room temperature of ≧950 MPa.
 11. The process as claimed in claim 1, wherein the shaped part has a modulus of elasticity in the range of from 290 to 320 GPa.
 12. The process as claimed in claim 1, wherein the shaped part has a coefficient of thermal expansion of 3.1×10⁻⁶ /K and a thermal conductivity of 20 W/m·K.
 13. The process as claimed in claim 1, wherein the Y₂ O₃ /Al₂ O₃ weight ratio is 2.0 in the shaped part.
 14. The process as claimed in claim 1, wherein the amount of HfO₂ is within the range of from 0.4 to 1% by weight in the shaped part.
 15. The process as claimed in claim 1, wherein the amount of ZrO₂ is within the range of from 0.2 to 1% by weight in the shaped part. 